100-Amp Sub-Panel Wire Size: NEC Feeder & Ground Sizing

Stand at the wire cage of any supply house on a Saturday and somebody will be holding a 100-amp breaker, asking which reel goes with it. Often they leave with 2-2-2-4 aluminum SER because the shelf tag says “100 amp” — and that cable ends up on a breaker it was never allowed to sit on.

The 100 amp sub panel wire size question trips people up because there are three different answers floating around and only one of them applies to a sub-panel. Service conductors get an 83% allowance. Cable gets held to a lower temperature column. And the next-size-up rule in 240.4(B) works for some conductor sizes but not others. Mix those up and the feeder is either undersized or written up.

This guide covers the four conductors in a 100-amp sub-panel feeder, the wire size for each wiring method, the step-by-step sizing through NEC 215.2, Table 310.16 and 110.14(C), when distance forces you to upsize, and the mistakes behind most failed sub-panel inspections.

What makes up a 100-amp sub-panel feeder

A sub-panel is a panelboard supplied by a feeder rather than by service conductors. The “100 amp” in the name describes the overcurrent device protecting that feeder in the upstream panel. The panelboard itself is often rated 125 or 200 amps, which is fine — NEC 408.36 only requires that the protective device not exceed the panelboard rating.

“What size wire for a 100 amp sub panel” is really four questions, because four conductors run between the panels and each one is sized by a different rule:

• Two ungrounded conductors. The hots. 3 AWG copper or 1 AWG aluminum, from the 75 °C column of Table 310.16, assuming 75 °C terminations at both ends.
• One grounded conductor. The neutral. Sized to the calculated unbalanced load under NEC 220.61, and never smaller than Table 250.122 requires (NEC 215.2(A)(2)). In practice it is full size.
• One equipment grounding conductor. 8 AWG copper or 6 AWG aluminum from Table 250.122, based on the 100-amp feeder breaker. Metal raceway may serve as the EGC under 250.118, but pull a wire anyway.
• A grounding electrode conductor — only if the sub-panel is in a detached building, where 250.32(A) requires its own electrode system. Size it from Table 250.66; for 3 AWG copper feeders that is 8 AWG copper, or 6 AWG where it runs only to ground rods.

The three-wire feeder to an outbuilding ended with 250.32(B)(1) in the 2008 NEC. Our guide to grounding an electrical panel covers the bonding side in detail.

Reading SER cable markings like 2-2-2-4 and 1-1-1-3

SER cable is sold by four numbers, and they read in order: hot – hot – neutral – ground. 1-1-1-3 aluminum SER is two 1 AWG ungrounded conductors, a 1 AWG insulated neutral and a 3 AWG bare ground. 2-2-2-4 is the same layout one size smaller. The first number decides the breaker — and a 2 there means 90 amps in aluminum.

100-amp sub-panel wire size by material and wiring method — NEC Table 310.16

Material and wiring method decide which column of Table 310.16 you may read:

The NM-B and SER rows are where the arguments start. NM-B is held to the 60 °C column by NEC 334.80, and the cable tops out at 2 AWG copper, which is 95 amps. Since 95 is not a standard breaker size, 240.4(B) lets you protect it at 100 amps — but only if the calculated load is 95 amps or less. Interior SER is different: 338.10(B)(4)(a) sends you to Part II of Article 334 excluding 334.80, so SER reads the 75 °C column.

That is exactly why 2-2-2-4 aluminum fails. At 75 °C, 2 AWG aluminum is 90 amps. The next-size-up rule only applies when the conductor ampacity does not match a standard rating in 240.6(A), and 90 amps is a standard rating. So the cable stops at a 90-amp breaker. If you want 100 amps out of SER, buy 1-1-1-3.

If you are wiring the service itself, the 83% numbers do apply — our guide to what size wire for a 100-amp service walks through Table 310.12(A) and the conditions that come with it.

Sizing the feeder — NEC 215.2, Table 310.16 and 110.14(C)

Feeder conductors are sized under NEC 215.2(A)(1) from Table 310.16, which assumes not more than three current-carrying conductors in a raceway or cable and a 30 °C (86 °F) ambient. These are the sizes on either side of a 100-amp decision. Our walkthrough of how to read NEC Table 310.16 covers the table itself in more depth.

Size	Copper 60 °C	Copper 75 °C	Aluminum 60 °C	Aluminum 75 °C
6 AWG	55 A	65 A	40 A	50 A
4 AWG	70 A	85 A	55 A	65 A
3 AWG	85 A	100 A	65 A	75 A
2 AWG	95 A	115 A	75 A	90 A
1 AWG	110 A	130 A	85 A	100 A
1/0 AWG	125 A	150 A	100 A	120 A
2/0 AWG	145 A	175 A	115 A	135 A
3/0 AWG	165 A	200 A	130 A	155 A

Step-by-step: sizing a 100-amp sub-panel feeder

Step 1 — calculate the load. Run the Article 220 calculation for everything the sub-panel will serve. Under 215.2(A)(1)(a) the conductor ampacity must be at least the noncontinuous load plus 125% of the continuous load. Anything running three hours or more is continuous.

Step 2 — pick the breaker. NEC 215.3 applies the same 125% test to the overcurrent device. A 100-amp breaker therefore supports 80 amps of continuous load.

Step 3 — check the terminations. NEC 110.14(C)(1)(a) holds circuits rated 100 amps or less, and conductors 14 through 1 AWG, to the 60 °C column unless the equipment is listed and marked for 75 °C. Nearly every modern breaker and load center is marked 60/75 °C, which puts you in the 75 °C column: 3 AWG copper or 1 AWG aluminum. On an older panel marked 60 °C only, the answer becomes 1 AWG copper (110 A) or 1/0 aluminum (100 A).

Step 4 — correct and adjust. Start from the 90 °C value of THWN-2 or XHHW-2 and apply the 310.15(B) ambient correction for a hot attic or a rooftop run, then the 310.15(C)(1) adjustment if more than three current-carrying conductors share the raceway. The neutral of a 120/240-volt feeder does not count, per 310.15(E)(1). The result still cannot exceed the 75 °C ampacity.

Step 5 — size the neutral and the ground. Neutral from the 220.61 unbalanced load, not smaller than Table 250.122. EGC from Table 250.122: 8 AWG copper or 6 AWG aluminum for a 100-amp device. See NEC 250.122 explained for the full table.

Step 6 — check the distance. Voltage drop is not a mandatory rule for most feeders, but the informational notes in 215.2(A) recommend no more than 3% on the feeder and 5% overall. Past roughly 180 feet at a realistic load, 3 AWG copper exceeds that — see the voltage drop table below.

What passes inspection — and what does not

Permitted

• 3 AWG copper THWN-2 in EMT, RMC, IMC or PVC — the default conduit feeder
• 1 AWG aluminum XHHW-2 or THWN-2 in conduit, with terminations listed for aluminum
• 1-1-1-3 aluminum SER run inside the building, read at the 75 °C column per 338.10(B)(4)(a)
• NM-B 2/3 in a dwelling where the calculated load is 95 amps or less, under 240.4(B)
• PVC with at least 18 inches of cover, or USE-2 / listed direct-burial cable with 24 inches of cover, per Table 300.5
• A main-breaker panel or other disconnect at a detached building, as required by 225.31, plus its own grounding electrode system under 250.32(A)

Not permitted

• 4 AWG copper or 2 AWG aluminum on a 100-amp sub-panel breaker using the 310.12 service allowance
• 2-2-2-4 aluminum SER on a 100-amp breaker
• The main bonding jumper or green bonding screw left installed in the sub-panel — 408.40 and 250.24(A)(5)
• A three-wire feeder (no EGC) to a detached building — 250.32(B)(1)
• Plain THHN in underground conduit, which 300.5(B) treats as a wet location — use THWN-2 or XHHW-2
• NM-B in buried conduit or any wet or damp location — 334.12(B)

Voltage drop on long 100-amp feeders

Detached shops and barns are where these feeders get long. The table gives the maximum one-way distance for 3% drop at 240 volts and 80 amps, the continuous limit of a 100-amp breaker.

Conductor	Area (cmil)	Max one-way length at 3%	EGC if upsized from base size
3 AWG copper	52,620	about 183 ft	8 AWG Cu (base size)
2 AWG copper	66,360	about 231 ft	6 AWG Cu
1 AWG copper	83,690	about 292 ft	4 AWG Cu (26,258 cmil needed)
1 AWG aluminum	83,690	about 177 ft	6 AWG Al (base size)
1/0 AWG aluminum	105,600	about 224 ft	4 AWG Al (33,110 cmil needed)
2/0 AWG aluminum	133,100	about 282 ft	4 AWG Al (41,732 cmil needed)

For a 150-foot run of 3 AWG copper at 80 amps the drop works out to 5.9 volts, or 2.5% — inside the recommendation. Our guide to voltage drop calculations covers the formula and the three-phase version. If you go to conduit, three 3 AWG THHN plus an 8 AWG EGC total 0.3285 in², just inside the 40% fill of 1-inch EMT; most electricians step up to 1-1/4 inch for an easier pull.

100-amp sub-panel vs 60, 125 and 200 amps

All values assume 75 °C terminations and no correction or adjustment:

Notice the jump between 100 and 125 amps: two copper sizes and three aluminum sizes for 25 more amps. That is why 100 amps is the sweet spot for most garages and shops. If the load calculation comes in well under 48 amps continuous, the 60-amp sub-panel costs less to wire. For the Article 215 rules behind all of these rows, see our summary of Article 215 feeders, and if you are choosing aluminum, read why aluminum connections fail before you torque the lugs.

Is a 100-amp sub-panel the right choice?

Wiring a 100-amp sub-panel in the field

Frequently asked questions

What size wire do I need for a 100 amp sub panel?

3 AWG copper or 1 AWG aluminum for the two hots, a full-size insulated neutral, and an 8 AWG copper or 6 AWG aluminum equipment grounding conductor. That assumes 75 °C terminations and a run under about 180 feet.

Can I use 4 AWG copper for a 100 amp sub panel?

No. 4 AWG copper is 85 amps at 75 °C. It is legal on a 100-amp dwelling service only because NEC 310.12 permits 83% sizing there, and that allowance does not extend to a feeder that carries only part of the dwelling load. The largest breaker 4 AWG copper can take on a sub-panel is 90 amps, under 240.4(B).

Is 2-2-2-4 aluminum SER good for 100 amps?

Not on a sub-panel feeder. 2 AWG aluminum is 90 amps at 75 °C, and because 90 amps is a standard breaker size the next-size-up rule does not apply. Use a 90-amp breaker, or buy 1-1-1-3 aluminum SER for a full 100 amps.

What size ground wire for a 100 amp sub panel?

8 AWG copper or 6 AWG aluminum per NEC Table 250.122, based on the 100-amp feeder breaker. If you upsize the ungrounded conductors for voltage drop, 250.122(B) requires the ground to increase in proportion.

Can I run Romex to a 100 amp sub panel?

Only inside a dwelling and only in the largest size. NM-B tops out at 2 AWG copper, which is 95 amps at the 60 °C column required by 334.80. It can sit on a 100-amp breaker under 240.4(B) if the calculated load is 95 amps or less. NM-B is never permitted in underground conduit or other wet locations.

How far can I run 3 AWG copper to a 100 amp sub panel?

About 180 feet one way keeps voltage drop at 3% with an 80-amp load on 240 volts. Beyond that, step up to 2 AWG or 1 AWG copper and increase the equipment grounding conductor under 250.122(B).

The bottom line

A 100-amp sub-panel is the right size for most garages, shops and additions, and the wire is simple once you separate it from the service rules: 3 AWG copper or 1 AWG aluminum, a full-size neutral, and an 8 AWG copper or 6 AWG aluminum ground. The two restraints that keep it legal are not borrowing the 83% sizing from 310.12, and not letting 240.4(B) talk you into 90-amp conductors on a 100-amp breaker.

Check the lug temperature rating, keep neutral and ground apart, and upsize the ground with the hots on a long run. For the conductor itself, our guide to THHN/THWN-2 wire covers which marking you need for underground and outdoor feeders.